..."I have also noticed that even rifles from Howa and Costa Mesa will show different tones to the uppers and lowers."...
i have looked ~and looked~ for an old magazine article about the 180 that specified the metals used in that model. NO CORNERS WERE CUT IN METAL SELECTION. strictly from memory- they used 4340, 4150, and even a 50 series, or 8620 for the bbl, all tool grade steels. the difference in alloys would explain the disimilar appearance of the parkerizing. look at the old "lead dipped" [annealed] ends of early parked garand rifles for comparison. these are even the same metal, but with a different hardness.
it also does the same to blueing, often considered a definite sign of a low quality job, but not absolutely true. in way of explaination, look at the purple hues of blueing on old 870 remington recievers...
below from:
http://www.harrisoncustom.com/TextDesc/Finishes.htm
"Parkerizing
Parkerizing has been around since before WW II and is still a process that delivers a useful finish today. The most common form of this finish is a deposit of manganese & phosphate, which are applied by soaking the parts in a heated bath. It must be done to parts that have been prepared by degreasing and sand or glass bead blasting. Parkerizing can only be done to carbon steel parts. The rule of thumb is if you can blue it, you should be able to Parkerize it. Parkerizing has a crystalline structure that will hold oils and waxes well. It varies in color between medium gray and charcoal black and has a sort of "soft" look to it. The old G.I. guns that had a greenish-gray color got that color from being coated with preservatives such as Cosmoline. The variation in color is due to differences in the base steel alloy and it's heat-treating and tempering. The color variation can occur on the same part, where a specific area was treated with induction hardening. A Parkerized gun is just a little more durable to abrasion than a blued gun, but is more rust resistant. Because it's been the finish on many of our military arms, it speaks to anyone that ever carried a U.S.G.I. M-1911."
any one who can find the article mentioned above, pls post it. i found it on the net about 4-5 years ago. {was a review from an old guns and ammo, i think} printed when the overall appearance of the rifle was very new and radical, so musta been a new product review.
gunnie